A conventional optical disc drive determines the number of data layers in an optical disc by means of a focus error signal. Please refer to FIG. 1, which is a diagram showing relative signals generated by the optical disc drive when detecting a DVD double layer disc. When the optical disc drive starts detecting data layers of the optical disc, the optical disc drive controls objectives lens of an optical pickup head upward and toward the optical disc, and a s-curve with smaller magnitude which represents a substrate layer appears. Afterward, if the focus error signal appears two s-curves, the optical disc drive thus determines that the detected optical disc is a double layer disc. On the contrary, if the focus error signal only appears a single s-curve, the optical disc drive determines the detected optical disc is a single layer disc.
However, the method for determining the number of data layers in an optical disc by means of s-curve fails easily when detecting a blu-ray disc. The reason why the conventional optical drive easily fails to determine the number of data layers in a blu-ray disc is described below.
In order to increase data density, the optical disc of an optical disc drive is equipped with a blu-ray (BD) laser in addition to a conventional CD laser and a conventional DVD laser. The wavelength λ of the BD laser is 405 nm while the wavelength λ of the DVD laser is 650 nm. Moreover, the numerical aperture NA of the BD laser is 0.85 while the numerical aperture NA of the DVD laser is 0.6. In other words, then BD laser has shorter wavelength and higher numerical aperture (NA) compared to the DVD laser. Please refer to FIG. 2, which is a table showing optical coefficients of the laser of an optical system and relative parameters. From the table we know, the spot size of the BD laser is smaller, data density is thus increased efficiently. However, the optical system is also more sensitive to the thickness variation of the optical disc Δd due to the design of the BD laser. The effect dueo to spherical aberration is proportional to four power of exponent of the numerical aperture NA. When the optical system adopts the BD laser reading/writing the optical disc, the effect due to spherical aberration cannot be neglected because the numerical aperture NA of the BD laser is higher.
Please refer to FIG. 3, which is a diagram showing the waveform of the practical focus error signal FE when a conventional optical disc drive reading a blu-ray double layer disc. When detecting a blu-ray disc, the quality of the focus error signal is getting worse due to the spherical aberration resulted from the thickness variation of each data layers in a blu-ray disc. The focus error signal is shrank and distorted as shown in dash line of the FIG. 3. The s-curve of the focus error signal is badly distorted. If the conventional method for determined the number of data layers is adopted, the optical disc drive will probably detect the blu-ray disc erroneously because the optical disc drive cannot recognize the actual number of s-curves which may represent the number of data layers in the optical disc. In addition, the blu-ray double layer disc or blu-ray multi-layer disc will be popularized because the requirement for 3D films. If the number of data layers in a blu-ray disc cannot be detected efficiently which results in the difficulties of adjusting servo signals, the disc reading speed will be delay and even read fail. Therefore, How to determine the number of data layers in a blu-ray disc efficiently is the subject matter of the present invention.